Glenn Mulcaire, the private investigator at the heart of the phone-hacking scandal, has been ordered to reveal who allegedly instructed him to intercept voicemails.

The supreme court ruled on Wednesday that Mulcaire must pass previously secret details about phone hacking to Nicola Phillips, the former assistant to Max Clifford.

Five senior law lords at Britain's highest court threw out Mulcaire's claim for privilege against self-incrimination, ending a 20-month legal appeal by the former private investigator over his "legitimate legal interests".

Mulcaire must now pass potentially incriminating details – including who at the News of the World allegedly instructed him to hack phones, the journalists he is alleged to have passed intercepted messages to, and how victims were allegedly targeted – to Phillips.

Judge Lord Walker said on Wednesday that questions of privilege against self-incrimination arose when an individual allegedly embarked on an offence which could be both a criminal action and a civil action.

He said the issue for the supreme court was to decide whether the content of allegedly intercepted voicemail messages amounted to intellectual property. He added that it was also up to the supreme court to decide whether conspiracy to intercept voicemail messages was a related charge that was likely to feature in any future court cases.

The private investigator had earlier challenged two rulings by the high court, on 17 November 2010 and 25 February 2011, and the court of appeal, on 1 February 2012, which found that the hacked voicemails were confidential, commercial and/or personal information and covered by section 72 of the Senior Courts Act.

Lord Walker said in the judgment: "The supreme court unanimously dismisses Mr Mulcaire's appeal. Section 72 of the [Senior Courts Act] excludes his privilege against self-incrimination: the proceedings brought by Ms Phillips are 'proceedings for … rights pertaining to …intellectual property' and the conspiracy proceedings to which Mr Mulcaire would expose himself on disclosure of the information amount to a 'related offence'."

The unanimous ruling was handed down at the supreme court by Lord Walker, Lord Kerr, Lord Clarke and Lord Dyson.

Mark Lewis, the solicitor for Phillips, said after the ruling that the disclosure of the information would take everything further on. He described the ruling as a precedent and said other potential civil litigants could now apply for the same information.

Mulcaire's solicitor, Sarah Webb of the law firm Payne Hicks Beach, said they had not yet decided whether to launch a further appeal.

Mulcaire, who was not in court for the verdict, said in a statement: "I am pleased that the supreme court has recognised that my right to refuse to answer questions which may incriminate me is more extensive than the court of appeal decided. As I made dear from the outset, I will comply with the supreme court's ruling to answer questions in Ms Phillips's case.

"The supreme court's decision, which is narrower than that of the court of appeal, will also determine whether there are any other cases where I have to give potentially incriminating answers to requests for information. I will consider with my lawyers what the wider implications of this judgment are if and when I am asked to answer such questions in other cases."

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